The Marine Corps said yesterday that it needs fewer of the mine-resistant armored vehicles hailed by military officials and policymakers as a possible solution to the problem of roadside bombs in Iraq and Afghanistan.

The Marines said they need 2,300 of the specialized vehicles, known as Mine Resistant Ambush Protected (MRAPs) or 1,400 fewer than the service estimated it needed earlier this year.

MRAPs, which can range from 40,000 to 70,000 pounds, depending on the style and maker, were considered to be one of the most effective defenses against roadside bombs, the biggest killer of U.S. troops in Iraq. Each has a raised chassis and V-shaped hull designed to deflect the impact of mines and other explosives.

But MRAPs are too big, too heavy and too hard to maneuver off-road, in confined areas or across bridges, said some Marine commanders who have used them. Heavily armored Humvees, while more vulnerable to explosives and bombs, are easier to maneuver, they said.

The Marines' decision to cut back comes as the number of roadside bomb attacks in Iraq has dramatically decreased over the past six months.

"I am completely comfortable with reducing our requirement for MRAPs based on input from Marine field commanders with experience employing these vehicles," Gen. James T. Conway, commandant of the Marine Corps, said in a statement. "This reduction in the total number of MRAPs does not put Marines at additional risk."

"They're awfully big and awfully heavy," said Raymond F. DuBois, a senior adviser at the Center for Strategic and International Studies and a former acting Army undersecretary. "That makes them an attractive target to the bad guys."

The Pentagon has said it was going to buy about 15,000 MRAPs, 3,700 of which were going to go to the Marine Corps, creating a program potentially worth $20 billion. The Pentagon chose more than a half-dozen companies to produce test versions of the MRAP, which can cost as much as $1 million each. Congress has pushed the Pentagon to hurry manufacturers to get the vehicles into combat zones.

The Marines are now using 500 MRAPs in combat zones and are expected to have an additional 100 by the end of the year.

The Corps has already contracted for the vehicles it wants, so its decision to cut back its order means it won't contract for any more. If its request is approved, it would save the Pentagon $1.7 billion, according to congressional officials.